Crystals of unliganded beta secretase and/or beta secretase-like proteins and the use thereof

ABSTRACT

The x-ray crystal structure of BACE or BACE-like proteins is useful for solving the structure of other molecules or molecular complexes, and identifying and/or designing potential modifiers of BACE activity.

This application claims the benefit of U.S. Provisional Application No.60/379,690, filed May 10, 2002, which is incorporated herein byreference in its entirety.

This application incorporates by reference the material contained on theduplicate (2) compact discs submitted herewith. Each disc contains thefollowing file:

Name Size Contents Date of File creation table_1.txt 268 kbytes Table 1May 7, 2002 table_2.txt 273 kbytes Table 2 Nov. 15, 2001

FIELD OF THE INVENTION

This invention relates to the crystallization and structuredetermination of beta secretase, also known as BACE OR Asp2.

BACKGROUND

Alzheimer's disease (AD) causes progressive dementia with consequentformation of amyloid plaques, neurofibrillary tangles, gliosis andneuronal loss. The disease occurs in both genetic and sporadic formswhose clinical course and pathological features are quite similar. Threegenes have been discovered to date which, when mutated, cause anautosomal dominant form of Alzheimer's disease. These encode the amyloidprotein precursor (APP) and two related proteins, presenilin-1 (PS1) andpresenilin-2 (PS2), which, as their names suggest, are structurally andfunctionally related. Mutations in any of the three proteins have beenobserved to enhance proteolytic processing of APP via an intracellularpathway that produces amyloid beta peptide (Aβ peptide, or sometimeshere as Abeta), a 40-42 amino acid long peptide that is the primarycomponent of amyloid plaque in AD.

Dysregulation of intracellular pathways for proteolytic processing maybe central to the pathophysiology of AD. In the case of plaqueformation, mutations in APP, PS1 or PS2 consistently alter theproteolytic processing of APP so as to enhance formation of Aβ 1-42, aform of the Aβ peptide which seems to be particularly amyloidogenic, andthus very important in AD. Different forms of APP range in size from695-770 amino acids, localize to the cell surface, and have a singleC-terminal transmembrane domain. The Abeta peptide is derived from aregion of APP adjacent to and containing a portion of the transmembranedomain. Normally, processing of APP at the α-secretase site cleaves themidregion of the Aβ sequence adjacent to the membrane and releases thesoluble, extracellular domain of APP from the cell surface. Thisα-secretase APP processing creates soluble APP-α, which is normal andnot thought to contribute to AD. Pathological processing of APP at theβ- and γ-secretase sites, which are located N-terminal and C-terminal tothe α-secretase site, respectively, produces a very different resultthan processing at the α site. Sequential processing at the β- andγ-secretase sites releases the Aβ peptide, a peptide possibly veryimportant in AD pathogenesis. Processing at the β- and γ-secretase sitescan occur in both the endoplasmic reticulum (in neurons) and in theendosomal/lysosomal pathway after reinternalization of cell surface APP(in all cells). Despite intense efforts, for 10 years or more, toidentify the enzymes responsible for processing APP at the β and γsites, to produce the Aβ peptide, those proteases remained unknown untilrecently. The identification and characterization of the β secretaseenzyme, termed Aspartyl Protease 2 (Asp2) has been established. Sincethe β-secretase catalyzes the committed step in formation of the Aβpeptide, it has become a key target in the search for therapeutic agentsto combat Alzheimer's disease. It is believed that inhibition of BACEshould slow or stop the onset of amyloid plaque formation and theassociated symptoms of Alzheimer's disease.

In addition, the X-ray crystal structure of human BACE in complex with apeptide inhibitor was solved and published (Hong et al., Science290:150-53 (2000)) from protein expressed in E. coli that contained nocovalent sugar (glycosylation) at any of the four putative glycosylationsites within the enzyme.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a crystal of betasecretase. In one embodiment the crystal includes unliganded betasecretase. In another embodiment, the crystal has orthorhombic spacecroup symmetry C222₁. In another embodiment, the crystal includes a unitcell defined by the dimensions a, b, c, α, β, and γ, wherein a is about55 Å to about 95 Å, b is about 84 Å to about 124 Å, c is about 80 Å toabout 120 Å, and α=β=γ=90°. Methods of using the crystal in a drugscreening assay are also provided.

In another embodiment, the present invention provides a method forcrystallizing a human beta secretase molecule. In one embodiment, themethod includes preparing purified human beta secretase in thesubstantial absence of a potential modifier, and crystallizing the humanbeta secretase from a solution having a pH of about 5.0 to about 5.6.

In another aspect, the present invention provides a method of preparinga crystal of liganded human beta secretase. In one embodiment, themethod includes exposing a crystal of unliganded human beta secretase toa fluid including a ligand. Preferably, the exposing includes soaking.

In another aspect, the present invention provides a method ofidentifying the ability for a potential ligand to bind to BACE. In oneembodiment, the method includes exposing a crystal of unliganded BACE toone or more samples including a potential ligand of BACE, anddetermining whether a ligand-BACE molecular complex is formed.Preferably the exposing includes soaking. Preferably, the one or moresamples include a plurality of potential ligands.

In another aspect, the present invention provides a method of acquiringstructural information for designing potential ligands for formingmolecular complexes with BACE. In one embodiment, the method includesexposing a crystal of unliganded BACE to a library of potential ligandshaving diverse shapes, and determining whether a ligand-BACE molecularcomplex is formed.

In another aspect, the present invention provides an unliganded moleculeincluding at least a portion of a human beta secretase or betasecretase-like binding pocket, wherein the binding pocket includes theamino acids listed in Table 3 and the binding pocket is defined by a setof points having a root mean square deviation of less than about 0.65 Åfrom points representing the backbone atoms of said amino acids asrepresented by the structure coordinates listed in Table 1.

In another aspect, the present invention provides a scalablethree-dimensional configuration of points, at least a portion of saidpoints derived from structure coordinates as listed in Table 1 of atleast a portion of an unliganded human beta secretase molecule thatincludes a human beta secretase or beta secretase-like binding pocket.Preferably, the scalable three-dimensional configuration of points aredisplayed as a holographic image, a stereodiagram, a model, or acomputer-displayed image.

In another aspect, the present invention provides a machine-readabledata storage medium including a data storage material encoded withmachine readable data which, when using a machine programmed withinstructions for using said data, displays a graphical three-dimensionalrepresentation of a molecule. Preferably the molecule is an unligandedmolecule including at least a portion of a human beta secretase or betasecretase-like binding pocket including the amino acids listed in Table3, the binding pocket defined by a set of points having a root meansquare deviation of less than about 0.65 Å from points representing thebackbone atoms of said amino acids as represented by structurecoordinates listed in Table 1 for a beta secretase or betasecretase-like molecule.

In another aspect, the present invention provides a method for obtainingstructural information about a molecule or a molecular complex ofunknown structure. In one embodiment, the method includes crystallizingthe molecule or molecular complex; generating an x-ray diffractionpattern from the crystallized molecule or molecular complex; andapplying to the x-ray diffraction pattern at least a portion of thestructure coordinates as set forth in Table 1 for human beta secretaseto generate a three-dimensional electron density map of at least aportion of the molecule or molecular complex whose structure is unknown.

In another aspect, the present invention provides a method for homologymodeling a human beta secretase homolog. In one embodiment, the methodincludes aligning the amino acid sequence of a human beta secretasehomolog with an amino acid sequence of human beta secretase andincorporating the sequence of the human beta secretase homolog into amodel of human beta secretase formed from structure coordinates as setforth in Table 1 for human beta secretase to yield a preliminary modelof the human beta secretase homolog; subjecting the preliminary model toenergy minimization to yield an energy minimized model; and remodelingregions of the energy minimized model where stereochemistry restraintsare violated to yield a final model of the human beta secretase homolog.

In another aspect, the present invention provides computer-assistedmethods for identifying, designing, or making a potential modifier ofhuman beta secretase activity. Preferably the methods include screeninga library of chemical entities.

ABBREVIATIONS

The following abbreviations may be used throughout this disclosure:Alzheimer's disease (AD)

-   3-[(1,1-Dimethylhydroxyethyl)amino]-2-hydroxy-1-propanesulfonic acid    (AMPSO)-   Amyloid beta peptide (Aβ peptide or Abeta)-   Amyloid protein precursor (APP)-   Aspartyl protease 2 (Asp2)-   Beta amyloid cleaving enzyme (BACE, memapsin 2, beta secretase)-   β-Mercaptoethanol (BME)-   3-Cyclohexylamino-1-propanesulfonic acid (CAPS)-   Dimethyl sulfoxide (DMSO)-   Ethylenediaminetetraacetic acid (EDTA)-   4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-   2-Methyl-2,4-pentanediol (MPD)-   4-Morpholineethanesulfonic acid (MES)-   Multiple anomalous dispersion (MAD)-   Presenilin-1 (PS1)-   Presenilin-2 (PS2)-   Poly(ethylene glycol) (PEG)-   2-Amino-2-hydroxymethyl-1,3-propanediol (TRIS)-   TE-TRIS-EDTA

The following amino acid abbreviations are used throughout thisdisclosure:

A = Ala = Alanine T = Thr = Threonine V = Val = Valine C = Cys =Cysteine L = Leu = Leucine Y = Tyr = Tyrosine I = Ile = Isoleucine N =Asn = Asparagine P = Pro = Proline Q = Gln = Glutamine F = Phe =Phenylalanine D = Asp = Aspartic Acid W = Trp = Tryptophan E = Glu =Glutamic Acid M = Met = Methionine K = Lys = Lysine G = Gly = Glycine R= Arg = Arginine S = Ser = Serine H = His = Histidine

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an illustration of the chemical structure of an inhibitor usedin co-crystallization experiments.

FIG. 2 is the synthetic peptideSer-Glu-Val-Asn-Sta-Val-Ala-Glu-Phe-Arg-Gly-Gly-Cys (where Sta=statine)(SEQ ID NO:2) used for affinity purification of BACE.

FIG. 3 illustrates a close-up view of the water bound to the catalyticaspartates (Asp 32 and 228) in the unliganded structure. This water isdisplaced when the inhibitor shown in FIG. 1 binds to the enzyme shownin the lower left.

FIG. 4 depicts three stereoviews comparing the liganded and unligandedforms of BACE. a) Overview of the entire molecule showing differenceslocalized to the flap and adjacent beta strands. b) A close-up view ofthe active site showing main chain backbone changes near the arrow. c)Similar view as (b) with the flap side chains visible near the arrow.

FIG. 5 depicts a comparison of active site surfaces in a) liganded andb) unliganded BACE. The ligand is the inhibitor shown in FIG. 1, and itis illustrated in b) for reference only.

FIG. 6 depicts the sequences of the E. Coli construct for recombinanthuman BACE (SEQ ID NO:1) used to obtain the crystals described. Thefirst visible residue in the crystal structures is indicated.

FIG. 7 is an illustration of the chemical structure of an inhibitor usedin a soaking experiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Tables 1 and 2 list atomic structure coordinates derived by x-raydiffraction of crystals having space groups C222₁ and C2, respectively,of human BACE expressed in E. coli. Column 2 lists a number for the atomin the structure. Column 3 lists the element whose coordinates aremeasured. The first letter in the column defines the element. Column 4lists the type of amino acid. Column 5 lists a number for the amino acidin the structure. Columns 6-8 list the crystallographic coordinates X,Y, and Z respectively. The crystallographic coordinates define theatomic position of the element measured. Column 9 lists an occupancyfactor that refers to the fraction of the molecules in which each atomoccupies the position specified by the coordinates. A value of “1”indicates that each atom has the same conformation, i.e., the sameposition, in all molecules of the crystal. Column 10 lists a thermalfactor “B” that measures movement of the atom around its atomic center.Column 11 lists the chain id (AA for molecule A in the asymmetric unit,BB for molecule B in the asymmetric unit, CC for molecule C in theasymmetric unit, WW for water molecules, and LL for inhibitormolecules). Column 12 lists the element whose coordinates are measured.

Crystalline Form(s) and Method of Making

The three-dimensional structure of human beta secretase was solved usingx-ray crystallography to 1.7 Å resolution for an unliganded betasecretase crystal. Preferably the beta secretase is isolated from E.coli. As used herein, “unliganded” means that the molecule does notinclude substantial amounts of active sites having a ligand complexedthereto. As used herein, “ligand” refers to a chemical entity that canform a reversible complex with the protein and that could function as adrug candidate (e.g., modifiers and inhibitors). Thus, the term “ligand”as used herein does not include chemical entities that could notfunction as a drug candidate (e.g., water, metal ions, and solvents).Preferably, an unliganded crystal includes at least about 70% unligandedactive sites, more preferably at least about 90% unliganded activesites, even more preferably at least about 95% unliganded active sites,and most preferably about 100% unliganded active sites.

Inspection of the active site revealed an absence of inhibitor in theelectron density map and a movement of the active site flap away fromthe rest of the protein molecule. This discovery led to the firstunliganded structure of BACE which was refined to an R-factor of 23.3%and a Free R-factor of 26.8% (Table 10). Details of this structure aredescribed herein.

Preferably, the crystal has orthorhombic space group symmetry C222₁.Preferably, the crystal includes orthorhombic shaped unit cells, eachunit cell having dimensions a=75±20 Å, b=104±20 Å, c=is 100±20 Å, andα=β=γ=90. More preferably, the crystal includes orthorhombic shaped unitcells, each unit cell having dimensions a=75±5 Å, b=˜104±5 Å, c=100±5 Å,and α=β=γ=90. Most preferably, the crystal includes tetragonal shapedunit cells, each unit cell having dimensions a=75 Å, b=104 Å, c=100 Å,and α=β=γ=90. Preferably, the crystallized enzyme is a monomer with onemonomer in the asymmetric unit. Preferably, human beta secretase isisolated from an E. coli strain, for example, from expression constructPQE70 with the propeptide removed and possessing a C-terminal His-6 tag.

In a preferred embodiment, protein is prepared either by firstconcentrating the protein in the presence of about a 2.4 molar excess ofabout 100% DMSO, or by the addition of about 1 mM to about 2 mM DMSO tothe concentrated protein sample (preferably about 6 mg/ml to about 8mg/ml).

Unliganded crystals are preferably grown in the substantial absence ofpotential modifiers (e.g., inhibitors), preferably using a PEG as aprecipitant. Preferably, the PEG has a number average molecular weightof at most about 10,000 Daltons, more preferably at most about 1,000Daltons, and most preferably at most about 500 Daltons. Preferably, thePEG has a number average molecular weight of at least about 100 Daltons,more preferably at least about 150 Daltons, and most preferably at leastabout 175 Daltons. A preferred PEG includes PEG 200. Preferably, atleast about 10% (w/v), and more preferably at least about 24% (w/v) PEG200 is used as precipitant, based on the total volume of solution.Preferably, at most about 50% (w/v), and more preferably at most about39% (w/v) PEG 200 is used as precipitant, based on the total volume ofsolution.

Preferably, the crystallization solution is buffered to about pH 5.0 toabout pH 5.6 using a buffer. Preferably, the buffer has a pK_(a) ofabout 3 to about 6. Preferred buffers include, for example, sodiumacetate. When sodium acetate is used as a buffer, it is preferably usedat a concentration of about 1 mM to about 200 mM. Typically, noadditional salt is present during crystallization. However, the solutionmay optionally include an additional salt, such as, for example, sodiumchloride, ammonium sulfate, magnesium sulfate, lithium sulfate, sodiumacetate, and combinations thereof. If the optional salt is present, thesolution preferably includes about 0.001 M to about 0.5 M of the salt.Optionally, the solution may include an organic solvent, preferably atmost about 40% by weight organic solvent. Preferred solvents include,for example, dimethylsulfoxide or 2-methyl-2,4-pentanediol.

Streak seeding at setup is preferred for optimal crystal growth. A catwhisker may be used to seed at setup with a thousand fold dilution ofseed stock. Crystals may be slowly grown over a number of days, forexample, 14 days. The crystal morphology preferably includes single rodswith an approximate size of 0.25×0.15×0.15 mm.

Variation in buffer and buffer pH as well as other additives such asPEG, PEG-MME, PEG-DME, or polyoxyalkylenepolyamines is apparent to thoseskilled in the art and may result in similar crystals.

The invention further includes a human beta secretase crystal that isisomorphous with a human beta secretase crystal having a unit celldefined by the dimensions of a, b, c, α, β, and γ, wherein a is about 55Å to about 95 Å, b is about 84 Å to about 124 Å, c is about 80 Å toabout 120 Å; and α=β=γ=90°

Soaking Protocol to Incorporate Ligands

The C222₁ unliganded crystal form led to several experiments to test itsutility in BACE-ligand molecular complex formation. As used herein, a“molecular complex” means a protein in covalent or non-covalentassociation with a chemical entity (e.g., a ligand). Crystal forms thatpossess large enough solvent channels can be used to form complexes bysoaking ligands into the crystal. Experiments with the inhibitor shownin FIG. 1 revealed that this ligand (e.g., an inhibitor) could besuccessfully added to the crystallization drop (after formation of theunliganded crystals) in order to form a complex with BACE. Various otherinhibitors of different levels of potency were tested in order to definethe parameters for soaking compounds into these unliganded crystals.

Addition of a stabilization solution containing about 1 mM to about 10mM of a BACE inhibitor (IC₅₀≦10 μM) to the crystallization drop providedconditions for formation of a BACE-inhibitor complex. No additionalcryoprotectant was needed as the crystallization conditions werecryogenic. The inhibitor shown in FIG. 1 was successfully soaked intothese unliganded crystals to form a complex consistent with earlierX-ray structures prepared by co-crystallization.

Routine use of the C222₁ crystal form for the preparation of BACE-ligandcomplexes significantly reduces the time between receipt of a compoundfor testing and generation of a complex. Because separateco-crystallization experiments are not required for each compound,complexes can be generated within a few days and subsequently data canbe collected. The higher symmetry of the C222₁ crystal form provides anadded advantage over the C2 crystal form in that 50% less data isrequired for a complete dataset reducing the time required for datacollection. There are occasions when soaking an inhibitor into the C222₁crystal form did not result in a BACE-inhibitor complex as expected dueto low solubility of the compound in the soaking conditions or otherunexplained factors. In these cases, the C2 crystal form provided analternative for complex preparation.

Crystallography may preferably be used to screen and identify chemicalentities that are not known ligands of target biomolecules as disclosed,for example, in U.S. Pat. No. 6,297,021 (Nienaber et al.). For example,crystallography may preferably be used to screen and identify chemicalentities that are not known ligands of BACE for their ability to bind toBACE. A preferred method includes obtaining a crystal of unligandedBACE; exposing the unliganded BACE to one or more test samples thatinclude a potential ligand of the BACE; and determining whether aligand-BACE molecular complex is formed. The BACE may be exposed topotential ligands by various methods including, for example, soaking aBACE crystal in a solution of one or more potential ligands, orco-crystallizing BACE in the presence of one or more potential ligands.

Structural information from the ligand-BACE complexes found maypreferably be used to design new ligands that bind tighter, bind morespecifically, have desired biological activity properties, have bettersafety profiles than known ligands, and combinations thereof. Forexample, libraries of “shape-diverse” chemical entities may preferablybe used to allow direct identification of the ligand-BACE complex evenwhen the ligand is exposed as part of a mixture. As used herein, “shapediverse” refers to ligands having substantial differences inthree-dimensional shapes that can be recognized, for example, by visualinspection of the two dimensional chemical structures, or by calculationand comparison of relevant parameters by a computational program. Shapediversity of the mixture permits a bound ligand to be identifieddirectly from the resultant electron density map. This preferably avoidsthe need for time-consuming deconvolution of a hit from the mixture.Here, three important steps are preferably achieved simultaneously.Preferably, the calculated electron density function directly revealsthe binding event, identifies the bound chemical entity, and provides adetailed 3-D structure of the ligand-BACE complex. Once a hit is found,preferably a number of analogs or derivatives of the hit may be screenedfor tighter binding or desired biological activity by traditionalscreening methods. Moreover, the identity of the hit and informationabout structure of the target may preferably be used to develop analogsor derivatives with tighter binding or desired biological activityproperties. Optionally, the ligand-BACE complex may be exposed toadditional iterations of potential ligands so that two or more hits maypreferably be linked together to identify or design a more potentligand.

X-Ray Crystallographic Analysis

Each of the constituent amino acids of human beta secretase is definedby a set of structure coordinates as set forth in Table 1. The term“structure coordinates” refers to Cartesian coordinates derived frommathematical equations related to the patterns obtained on diffractionof a monochromatic beam of x-rays by the atoms (scattering centers) of ahuman beta secretase complex in crystal form. The diffraction data areused to calculate an electron density map of the repeating unit of thecrystal. The electron density maps are then used to establish thepositions of the individual atoms of the human beta secretase protein orprotein/ligand complex.

Slight variations in structure coordinates can be generated bymathematically manipulating the human beta secretase or human betasecretase/ligand structure coordinates. For example, the structurecoordinates set forth in Table 1 could be manipulated bycrystallographic permutations of the structure coordinates,fractionalization of the structure coordinates, integer additions orsubtractions to sets of the structure coordinates, inversion of thestructure coordinates or any combination of the above. Alternatively,modifications in the crystal structure due to mutations, additions,substitutions, and/or deletions of amino acids, or other changes in anyof the components that make up the crystal, could also yield variationsin structure coordinates. Such slight variations in the individualcoordinates will have little effect on overall shape. If such variationsare within an acceptable standard error as compared to the originalcoordinates, the resulting three-dimensional shape is considered to bestructurally equivalent. Structural equivalence is described in moredetail below.

It should be noted that slight variations in individual structurecoordinates of the human beta secretase would not be expected tosignificantly alter the nature of chemical entities such as ligands thatcould associate with the binding pockets. In this context, the phrase“associating with” refers to a condition of proximity between a chemicalentity, or portions thereof, and a human beta secretase molecule orportions thereof. The association may be non-covalent, wherein thejuxtaposition is energetically favored by hydrogen bonding, van derWaals forces, or electrostatic interactions, or it may be covalent.Thus, for example, a ligand that bound to a binding pocket of human betasecretase would also be expected to bind to or interfere with astructurally equivalent binding pocket.

For the purpose of this invention, any molecule or molecular complex orbinding pocket thereof, or any portion thereof, that has a root meansquare deviation of conserved residue backbone atoms (N, Cα, C, O) ofless than about 0.65 Å, when superimposed on the relevant backbone atomsdescribed by the reference structure coordinates listed in Table 1, isconsidered “structurally equivalent” to the reference molecule. That isto say, the crystal structures of those portions of the two moleculesare substantially identical, within acceptable error. As used herein,“residue” refers to one or more atoms. Particularly preferredstructurally equivalent molecules or molecular complexes are those thatare defined by the entire set of structure coordinates listed in Table1± a root mean square deviation from the conserved backbone atoms ofthose amino acids of less than about 0.65 Å. More preferably, the rootmean square deviation is at most about 0.5 Å, and even more preferably,at most about 0.35 Å. Other embodiments of this invention include amolecular complex defined by the structure coordinates listed in Table 1for those amino acids listed in Table 3, Table 4, or Table 5, ± a rootmean square deviation from the conserved backbone atoms of those aminoacids of less than about 0.65 Å, preferably at most about 0.5 Å, andmore preferably at most about 0.35 Å.

The term “root mean square deviation” means the square root of thearithmetic mean of the squares of the deviations. It is a way to expressthe deviation or variation from a trend or object. For purposes of thisinvention, the “root mean square deviation” defines the variation in thebackbone of a protein from the backbone of human beta secretase or abinding pocket portion thereof, as defined by the structure coordinatesof human beta secretase described herein.

It will be readily apparent to those of skill in the art that thenumbering of amino acids in other isoforms of human beta secretase maybe different than that of human beta secretase expressed in E. coli.

Active Site and Other Structural Features

Applicants' invention provides information about the shape and structureof the binding pocket of human beta secretase in the presence of apotential modifier. The secondary structure of the human beta secretasemonomer includes two domains consistent with a typical aspartic proteasefold.

Binding pockets are of significant utility in fields such as drugdiscovery. The association of natural ligands or substrates with thebinding pockets of their corresponding receptors or enzymes is the basisof many biological mechanisms of action. Similarly, many drugs exerttheir biological effects through association with the binding pockets ofreceptors and enzymes. Such associations may occur with all or any partsof the binding pocket. An understanding of such associations helps leadto the design of drugs having more favorable associations with theirtarget, and thus improved biological effects. Therefore, thisinformation is valuable in designing potential modifiers of betasecretase-like binding pockets, as discussed in more detail below.

The term “binding pocket,” as used herein, refers to a region of amolecule or molecular complex, that, as a result of its shape, favorablyassociates with another chemical entity. Thus, a binding pocket mayinclude or consist of features such as cavities, surfaces, or interfacesbetween domains. Chemical entities that may associate with a bindingpocket include, but are not limited to, cofactors, substrates,modifiers, agonists, and antagonists.

The amino acid constituents of a human beta secretase binding pocket asdefined herein are positioned in three dimensions in accordance with thestructure coordinates listed in Table 1 and/or Table 2. In one aspect,the structure coordinates defining a binding pocket of human betasecretase include structure coordinates of all atoms in the constituentamino acids; in another aspect, the structure coordinates of a bindingpocket include structure coordinates of just the backbone atoms of theconstituent amino acids.

The binding pocket of human beta secretase preferably includes the aminoacids listed in Table 3, more preferably the amino acids listed in Table4, and most preferably the amino acids listed in Table 5, as representedby the structure coordinates listed in Table 1 and/or Table 2.Alternatively, the binding pocket of human beta secretase may be definedby those amino acids whose backbone atoms are situated within about 4 Å,more preferably within about 7 Å, most preferably within about 10 Å, ofone or more constituent atoms of a bound substrate or modifier. In yetanother alternative, the binding pocket may be defined by those aminoacids whose backbone atoms are situated within a sphere centered on thecoordinates representing the alpha carbon atom of residue Thr 231, thesphere having a radius of about 15 Å, preferably about 20 Å, and morepreferably about 25 Å.

The term “beta secretase-like binding pocket” refers to a portion of amolecule or molecular complex whose shape is sufficiently similar to atleast a portion of a binding pocket of human beta secretase as to beexpected to bind related structural analogues. As used herein, “at leasta portion” means that at least about 50% of the amino acids areincluded, preferably at least about 70% of the amino acids are included,more preferably at least about 90% of the amino acids are included, andmost preferably all the amino acids are included. A structurallyequivalent binding pocket is defined by a root mean square deviationfrom the structure coordinates of the backbone atoms of the amino acidsthat make up binding pockets in human beta secretase (as set forth inTable 1) of at most about 0.35 Å. How this calculation is obtained isdescribed below.

Accordingly, the invention provides molecules or molecular complexesincluding a human beta secretase binding pocket or beta secretase-likebinding pocket, as defined by the sets of structure coordinatesdescribed above.

TABLE 3 Residues with 4Å of a binding site. GLY 11 GLY 12 GLY 13 LEU 30ASP 32 GLY 34 SER 35 PRO 70 TYR 71 THR 72 GLN 73 GLY 74 PHE 108 ILE 110TRP 115 TYR 198 ASP 228 GLY 230 THR 231 THR 232 ARG 235

TABLE 4 Residues with 7Å of a binding site. LYS 9 GLY 11 GLN 12 GLY 13TYR 14 LEU 30 ASP 32 THR 33 GLY 34 SER 35 SER 36 ASN 37 VAL 69 PRO 70TYR 71 THR 72 GLN 73 GLY 74 LYS 75 TRP 76 ASP 106 LYS 107 PHE 108 PHE109 ILE 110 TRP 115 ILE 118 ILE 126 ALA 127 ARG 128 TYR 198 LYS 224 ILE226 ASP 228 SER 229 GLY 230 THR 231 THR 232 ASN 233 ARG 235 SER 325 GLN326 THR 329 VAL 332 ALA 335

TABLE 5 Residues with 10Å of a binding site. ARG 7 GLY 8 LYS 9 SER 10GLY 11 GLN 12 GLY 13 TYR 14 TYR 15 ILE 29 LEU 30 VAL 31 ASP 32 THR 33GLY 34 SER 35 SER 36 ASN 37 PHE 38 PHE 47 TYR 68 VAL 69 PRO 70 TYR 71THR 72 GLN 73 GLY 74 LYS 75 TRP 76 ILE 102 SER 105 ASP 106 LYS 107 PHE108 PHE 109 ILE 110 ASN 111 SER 113 ASN 114 TRP 115 GLU 116 GLY 117 ILE118 LEU 119 GLY 120 LEU 121 ALA 122 TYR 123 ALA 124 GLU 125 ILE 126 ALA127 ARG 128 PRO 129 LEU 154 LEU 167 VAL 170 TRP 197 TYR 198 TYR 199 ASP223 LYS 224 SER 225 ILE 226 VAL 227 ASP 228 SER 229 GLY 230 THR 231 THR232 ASN 233 LEU 234 ARG 235 LEU 236 GLY 264 ARG 307 LYS 321 ALA 323 ILE324 SER 325 GLN 326 SER 327 SER 328 THR 329 GLY 330 THR 331 VAL 332 MET333 GLY 334 ALA 335 VAL 336 GLU 339Overview of Liganded and Unliganded Structures

The structure of BACE was reported in the literature in October 2000(Hong et al., Science 290:150-53 (2000)) and made available through theProtein Data Bank (id code: 1FKN). BACE is comprised of two domains thatcorrespond to the N- and C-terminal portions of the protein. The proteinfold is similar to the structures of other aspartic proteases solved byX-ray crystallography. The secondary structure of the protein core isprimarily beta sheet with several alpha helices on the surface of theprotein. The 1FKN structure and the structure solved herein do notcontain the transmembrane domain found in the wildtype protein. Thisdomain has been removed for ease of protein expression, purification,and crystallization.

The inhibitor shown in FIG. 1 sits at the active site between thecatalytic aspartates (residues 32 and 228) and the flap (residues66-76). In the liganded structure the flap is closed down over theligand making a single hydrogen bond and several van der Waals contacts.The presence of the ligand displaces a bound water molecule observed inthe unliganded structure to interact directly with the catalyticaspartates (FIG. 3). In the unliganded structure, the flap (residues66-76) and the adjacent beta strands (residues 101-107 and 42-47) moveaway from the core of the protein (FIG. 4). The Cα of Gln 73 moves 5.9 Åaway from its position in the liganded structure, while the Cα of Thr72moves 3.7 Å away from its position in the liganded structure. The flapundergoes not only movement away from the active site but also atwisting motion which moves the left hand portion of the flap furtheraway from the active site. The result of this motion is a more openactive site and a change in the topology of the P1 and P2′ pockets.

Three-Dimensional Configurations

X-ray structure coordinates define a unique configuration of points inspace. Those of skill in the art understand that a set of structurecoordinates for protein or an protein/ligand complex, or a portionthereof, define a relative set of points that, in turn, define aconfiguration in three dimensions. A similar or identical configurationcan be defined by an entirely different set of coordinates, provided thedistances and angles between coordinates remain essentially the same. Inaddition, a scalable configuration of points can be defined byincreasing or decreasing the distances between coordinates by a scalarfactor while keeping the angles essentially the same.

The present invention thus includes the scalable three-dimensionalconfiguration of points derived from the structure coordinates of atleast a portion of a human beta secretase molecule or molecular complex,as listed in Table 1, as well as structurally equivalent configurations,as described below. Preferably, the scalable three-dimensionalconfiguration includes points derived from structure coordinatesrepresenting the locations of a plurality of the amino acids defining ahuman beta secretase binding pocket.

In one embodiment, the scalable three-dimensional configuration includespoints derived from structure coordinates representing the locations thebackbone atoms of a plurality of amino acids defining the human betasecretase binding pocket, preferably the amino acids listed in Table 3,more preferably the amino acids listed in Table 4, and most preferablythe amino acids listed in Table 5. Alternatively, the scalablethree-dimensional configuration includes points derived from structurecoordinates representing the locations of the side chain and thebackbone atoms (other than hydrogens) of a plurality of the amino acidsdefining the human beta secretase binding pocket, preferably the aminoacids listed in Table 3, more preferably the amino acids listed in Table4, and most preferably the amino acids listed in Table 5.

Likewise, the invention also includes the scalable three-dimensionalconfiguration of points derived from structure coordinates of moleculesor molecular complexes that are structurally homologous to betasecretase, as well as structurally equivalent configurations.Structurally homologous molecules or molecular complexes are definedbelow. Advantageously, structurally homologous molecules can beidentified using the structure coordinates of human beta secretaseaccording to a method of the invention.

The configurations of points in space derived from structure coordinatesaccording to the invention can be visualized as, for example, aholographic image, a stereodiagram, a model, or a computer-displayedimage, and the invention thus includes such images, diagrams or models.

Structurally Equivalent Crystal Structures

Various computational analyses can be used to determine whether amolecule or a binding pocket portion thereof is “structurallyequivalent,” defined in terms of its three-dimensional structure, to allor part of human beta secretase or its binding pockets. Such analysesmay be carried out in current software applications, such as theMolecular Similarity application of QUANTA (Molecular Simulations Inc.,San Diego, Calif.) version 4.1, and as described in the accompanyingUser's Guide.

The Molecular Similarity application permits comparisons betweendifferent structures, different conformations of the same structure, anddifferent parts of the same structure. The procedure used in MolecularSimilarity to compare structures is divided into four steps: (1) loadthe structures to be compared; (2) define the atom equivalences in thesestructures; (3) perform a fitting operation; and (4) analyze theresults.

Each structure is identified by a name. One structure is identified asthe target (i.e., the fixed structure); all remaining structures areworking structures (i.e., moving structures). Since atom equivalencywithin QUANTA is defined by user input, for the purpose of thisinvention equivalent atoms are defined as protein backbone atoms (N, Cα,C, and O) for all conserved residues between the two structures beingcompared. A conserved residue is defined as a residue which isstructurally or functionally equivalent. Only rigid fitting operationsare considered.

When a rigid fitting method is used, the working structure is translatedand rotated to obtain an optimum fit with the target structure. Thefitting operation uses an algorithm that computes the optimumtranslation and rotation to be applied to the moving structure, suchthat the root mean square difference of the fit over the specified pairsof equivalent atom is an absolute minimum. This number, given inangstroms, is reported by QUANTA.

Machine Readable Storage Media

Transformation of the structure coordinates for all or a portion ofhuman beta secretase or the human beta secretase/ligand complex or oneof its binding pockets, for structurally homologous molecules as definedbelow, or for the structural equivalents of any of these molecules ormolecular complexes as defined above, into three-dimensional graphicalrepresentations of the molecule or complex can be conveniently achievedthrough the use of commercially-available software.

The invention thus further provides a machine-readable storage mediumincluding a data storage material encoded with machine readable datawhich, when using a machine programmed with instructions for using saiddata, displays a graphical three-dimensional representation of any ofthe molecule or molecular complexes of this invention that have beendescribed above. In a preferred embodiment, the machine-readable datastorage medium includes a data storage material encoded with machinereadable data which, when using a machine programmed with instructionsfor using said data, displays a graphical three-dimensionalrepresentation of a molecule or molecular complex including all or anyparts of a human beta secretase binding pocket or an beta secretase-likebinding pocket, as defined above. In another preferred embodiment, themachine-readable data storage medium includes a data storage materialencoded with machine readable data which, when using a machineprogrammed with instructions for using said data, displays a graphicalthree-dimensional representation of a molecule or molecular complexdefined by the structure coordinates of all of the amino acids listed inTable 1, ± a root mean square deviation from the backbone atoms of saidamino acids of less than about 0.65 Å, more preferably at most about 0.5Å, and even more preferably, at most about 0.35 Å.

In an alternative embodiment, the machine-readable data storage mediumincludes a data storage material encoded with a first set of machinereadable data which includes the Fourier transform of the structurecoordinates set forth in Table 1, and which, when using a machineprogrammed with instructions for using said data, can be combined with asecond set of machine readable data including the x-ray diffractionpattern of a molecule or molecular complex to determine at least aportion of the structure coordinates corresponding to the second set ofmachine readable data.

For example, a system for reading a data storage medium may include acomputer including a central processing unit (“CPU”), a working memorywhich may be, e.g., RAM (random access memory) or “core” memory, massstorage memory (such as one or more disk drives or CD-ROM drives), oneor more display devices (e.g., cathode-ray tube (“CRT”) displays, lightemitting diode (“LED”) displays, liquid crystal displays (“LCDs”),electroluminescent displays, vacuum fluorescent displays, field emissiondisplays (“FEDs”), plasma displays, projection panels, etc.), one ormore user input devices (e.g., keyboards, microphones, mice, trackballs, touch pads, etc.), one or more input lines, and one or moreoutput lines, all of which are interconnected by a conventionalbidirectional system bus. The system may be a stand-alone computer, ormay be networked (e.g., through local area networks, wide area networks,intranets, extranets, or the internet) to other systems (e.g.,computers, hosts, servers, etc.). The system may also include additionalcomputer controlled devices such as consumer electronics and appliances.

Input hardware may be coupled to the computer by input lines and may beimplemented in a variety of ways. Machine-readable data of thisinvention may be inputted via the use of a modem or modems connected bya telephone line or dedicated data line. Alternatively or additionally,the input hardware may include CD-ROM drives or disk drives. Inconjunction with a display terminal, a keyboard may also be used as aninput device.

Output hardware may be coupled to the computer by output lines and maysimilarly be implemented by conventional devices. By way of example, theoutput hardware may include a display device for displaying a graphicalrepresentation of a binding pocket of this invention using a programsuch as QUANTA as described herein. Output hardware might also include aprinter, so that hard copy output may be produced, or a disk drive, tostore system output for later use.

In operation, a CPU coordinates the use of the various input and outputdevices, coordinates data accesses from mass storage devices, accessesto and from working memory, and determines the sequence of dataprocessing steps. A number of programs may be used to process themachine-readable data of this invention. Such programs are discussed inreference to the computational methods of drug discovery as describedherein. References to components of the hardware system are included asappropriate throughout the following description of the data storagemedium.

Machine-readable storage devices useful in the present inventioninclude, but are not limited to, magnetic devices, electrical devices,optical devices, and combinations thereof. Examples of such data storagedevices include, but are not limited to, hard disk devices, CD devices,digital video disk devices, floppy disk devices, removable hard diskdevices, magneto-optic disk devices, magnetic tape devices, flash memorydevices, bubble memory devices, holographic storage devices, and anyother mass storage peripheral device. It should be understood that thesestorage devices include necessary hardware (e.g., drives, controllers,power supplies, etc.) as well as any necessary media (e.g., disks, flashcards, etc.) to enable the storage of data.

Structurally Homologous Molecules, Molecular Complexes, and CrystalStructures

The structure coordinates set forth in Table 1 can be used to aid inobtaining structural information about another crystallized molecule ormolecular complex. The method of the invention allows determination ofat least a portion of the three-dimensional structure of molecules ormolecular complexes which contain one or more structural features thatare similar to structural features of human beta secretase. Thesemolecules are referred to herein as “structurally homologous” to humanbeta secretase. Similar structural features can include, for example,regions of amino acid identity, conserved active site or binding sitemotifs, and similarly arranged secondary structural elements (e.g., αhelices and β sheets). Optionally, structural homology is determined byaligning the residues of the two amino acid sequences to optimize thenumber of identical amino acids along the lengths of their sequences;gaps in either or both sequences are permitted in making the alignmentin order to optimize the number of identical amino acids, although theamino acids in each sequence must nonetheless remain in their properorder. Preferably, two amino acid sequences are compared using theBlastp program, version 2.0.9, of the BLAST 2 search algorithm, asdescribed by Tatusova et al., FEMS Microbiol Lett., 174:247-50 (1999).Preferably, the default values for all BLAST 2 search parameters areused, including matrix=BLOSUM62; open gap-penalty=11, extension gappenalty=1, gap x_dropoff=50, expect=10, wordsize=3, and filter on. Inthe comparison of two amino acid sequences using the BLAST searchalgorithm, structural similarity is referred to as “identity.”Preferably, a structurally homologous molecule is a protein that has anamino acid sequence sharing at least 65% identity with a native orrecombinant amino acid sequence of human beta secretase (for example,SEQ ID NO:1). More preferably, a protein that is structurally homologousto human beta secretase includes a contiguous stretch of at least 50amino acids that shares at least 80% amino acid sequence identity withthe analogous portion of the native or recombinant human beta secretase(for example, SEQ ID NO:1). Methods for generating structuralinformation about the structurally homologous molecule or molecularcomplex are well-known and include, for example, molecular replacementtechniques.

Therefore, in another embodiment this invention provides a method ofutilizing molecular replacement to obtain structural information about amolecule or molecular complex whose structure is unknown including thesteps of:

(a) crystallizing the molecule or molecular complex of unknownstructure;

(b) generating an x-ray diffraction pattern from said crystallizedmolecule or molecular complex; and

(c) applying at least a portion of the structure coordinates set forthin Table 1 to the x-ray diffraction pattern to generate athree-dimensional electron density map of the molecule or molecularcomplex whose structure is unknown.

By using molecular replacement, all or part of the structure coordinatesof human beta secretase or the human beta secretase/ligand complex asprovided by this invention can be used to determine the structure of acrystallized molecule or molecular complex whose structure is unknownmore quickly and efficiently than attempting to determine suchinformation ab initio.

Molecular replacement provides an accurate estimation of the phases foran unknown structure. Phases are a factor in equations used to solvecrystal structures that cannot be determined directly. Obtainingaccurate values for the phases, by methods other than molecularreplacement, is a time-consuming process that involves iterative cyclesof approximations and refinements and greatly hinders the solution ofcrystal structures. However, when the crystal structure of a proteincontaining at least a structurally homologous portion has been solved,the phases from the known structure provide a satisfactory estimate ofthe phases for the unknown structure.

Thus, this method involves generating a preliminary model of a moleculeor molecular complex whose structure coordinates are unknown, byorienting and positioning the relevant portion of human beta secretaseor the human beta secretase/modifier complex within the unit cell of thecrystal of the unknown molecule or molecular complex so as best toaccount for the observed x-ray diffraction pattern of the crystal of themolecule or molecular complex whose structure is unknown. Phases canthen be calculated from this model and combined with the observed x-raydiffraction pattern amplitudes to generate an electron density map ofthe structure whose coordinates are unknown. This, in turn, can besubjected to any well-known model building and structure refinementtechniques to provide a final, accurate structure of the unknowncrystallized molecule or molecular complex (Lattman, Meth. Enzymol.,115, 55-77 (1985); M. G. Rossman, ed., “The Molecular ReplacementMethod,” Int. Sci. Rev. Ser., No. 13, Gordon & Breach, New York (1972)).

Structural information about a portion of any crystallized molecule ormolecular complex that is sufficiently structurally homologous to aportion of human beta secretase can be resolved by this method. Inaddition to a molecule that shares one or more structural features withhuman beta secretase as described above, a molecule that has similarbioactivity, such as the same catalytic activity, substrate specificityor ligand binding activity as human beta secretase, may also besufficiently structurally homologous to human beta secretase to permituse of the structure coordinates of human beta secretase to solve itscrystal structure.

In a preferred embodiment, the method of molecular replacement isutilized to obtain structural information about a molecule or molecularcomplex, wherein the molecule or molecular complex includes a human betasecretase subunit or homolog. A “subunit” of human beta secretase is ahuman beta secretase molecule that has been truncated at the N-terminusor the C-terminus, or both. In the context of the present invention, a“homolog” of human beta secretase is a protein that contains one or moreamino acid substitutions, deletions, additions, or rearrangements withrespect to the amino acid sequence of human beta secretase (SEQ IDNO:1), but that, when folded into its native conformation, exhibits oris reasonably expected to exhibit at least a portion of the tertiary(three-dimensional) structure of human beta secretase. For example,structurally homologous molecules can contain deletions or additions ofone or more contiguous or noncontiguous amino acids, such as a loop or adomain. Structurally homologous molecules also include “modified” humanbeta secretase molecules that have been chemically or enzymaticallyderivatized at one or more constituent amino acid, including side chainmodifications, backbone modifications, and N- and C-terminalmodifications including acetylation, hydroxylation, methylation,amidation, and the attachment of carbohydrate or lipid moieties,cofactors, and the like.

A heavy atom derivative of human beta secretase is also included as ahuman beta secretase homolog. The term “heavy atom derivative” refers toderivatives of human beta secretase produced by chemically modifying acrystal of human beta secretase. In practice, a crystal is soaked in asolution containing heavy metal atom salts, or organometallic compounds,e.g., lead chloride, gold thiomalate, thiomersal or uranyl acetate,which can diffuse through the crystal and bind to the surface of theprotein. The location(s) of the bound heavy metal atom(s) can bedetermined by x-ray diffraction analysis of the soaked crystal. Thisinformation, in turn, is used to generate the phase information used toconstruct three-dimensional structure of the protein (T. L. Blundell andN. L. Johnson, Protein Crystallography, Academic Press (1976)).

Because human beta secretase can crystallize in more than one crystalform, the structure coordinates of human beta secretase as provided bythis invention are particularly useful in solving the structure of othercrystal forms of human beta secretase or human beta secretase complexes.

The structure coordinates of human beta secretase as provided by thisinvention are particularly useful in solving the structure of human betasecretase mutants. Mutants may be prepared, for example, by expressionof human beta secretase cDNA previously altered in its coding sequenceby oligonucleotide-directed mutagenesis. Mutants may also be generatedby site-specific incorporation of unnatural amino acids into betasecretase proteins using the general biosynthetic method of Noren etal., Science, 244:182-88 (1989). In this method, the codon encoding theamino acid of interest in wild-type human beta secretase is replaced bya “blank” nonsense codon, TAG, using oligonucleotide-directedmutagenesis. A suppressor tRNA directed against this codon is thenchemically aminoacylated in vitro with the desired unnatural amino acid.The aminoacylated tRNA is then added to an in vitro translation systemto yield a mutant human beta secretase with the site-specificincorporated unnatural amino acid.

Selenocysteine or selenomethionine may be incorporated into wild-type ormutant human beta secretase by expression of human betasecretase-encoding cDNAs in auxotrophic E. coli strains (Hendrickson etal., EMBO J, 9:1665-72 (1990)). In this method, the wild-type ormutagenized human beta secretase cDNA may be expressed in a hostorganism on a growth medium depleted of either natural cysteine ormethionine (or both) but enriched in selenocysteine or selenomethionine(or both). Alternatively, selenomethionine analogues may be prepared bydown regulation methionine biosynthesis. (Benson et al., Nat. Struct.Biol., 2:644-53 (1995); Van Duyne et al., J. Mol. Biol., 229:105-24(1993)).

The structure coordinates of human beta secretase listed in Table 1 arealso particularly useful to solve the structure of crystals of humanbeta secretase, human beta secretase mutants or human beta secretasehomologs co-complexed with a variety of chemical entities. This approachenables the determination of the optimal sites for interaction betweenchemical entities, including candidate human beta secretase modifiersand human beta secretase. Potential sites for modification within thevarious binding sites of the molecule can also be identified. Thisinformation provides an additional tool for determining the mostefficient binding interactions, for example, increased hydrophobicinteractions, between human beta secretase and a chemical entity. Forexample, high resolution x-ray diffraction data collected from crystalsexposed to different types of solvent allows the determination of whereeach type of solvent molecule resides. Small molecules that bind tightlyto those sites can then be designed and synthesized and tested for theirpotential human beta secretase inhibition activity.

All of the complexes referred to above may be studied using well-knownx-ray diffraction techniques and may be refined versus 1.5-3.5 Åresolution x-ray data to an R value of about 0.30 or less using computersoftware, such as X-PLOR (Yale University, 81992, distributed byMolecular Simulations, Inc.; see, e.g., Blundell & Johnson, supra; Meth.Enzymol., Vol. 114 & 115, H. W. Wyckoff et al., eds., Academic Press(1985)). This information may thus be used to optimize known human betasecretase modifiers, and more importantly, to design new human betasecretase modifiers.

The invention also includes the unique three-dimensional configurationdefined by a set of points defined by the structure coordinates for amolecule or molecular complex structurally homologous to human betasecretase as determined using the method of the present invention,structurally equivalent configurations, and magnetic storage mediaincluding such set of structure coordinates.

Further, the invention includes structurally homologous molecules asidentified using the method of the invention.

Homology Modeling

Using homology modeling, a computer model of a human beta secretasehomolog can be built or refined without crystallizing the homolog.First, a preliminary model of the human beta secretase homolog iscreated by sequence alignment with human beta secretase, secondarystructure prediction, the screening of structural libraries, or anycombination of those techniques. Computational software may be used tocarry out the sequence alignments and the secondary structurepredictions. Structural incoherences, e.g., structural fragments aroundinsertions and deletions, can be modeled by screening a structurallibrary for peptides of the desired length and with a suitableconformation. For prediction of the side chain conformation, a sidechain rotamer library may be employed. If the human beta secretasehomolog has been crystallized, the final homology model can be used tosolve the crystal structure of the homolog by molecular replacement, asdescribed above. Next, the preliminary model is subjected to energyminimization to yield an energy minimized model. The energy minimizedmodel may contain regions where stereochemistry restraints are violated,in which case such regions are remodeled to obtain a final homologymodel. The homology model is positioned according to the results ofmolecular replacement, and subjected to further refinement includingmolecular dynamics calculations.

Rational Drug Design

Computational techniques can be used to screen, identify, select and/ordesign chemical entities capable of associating with human betasecretase or structurally homologous molecules. Knowledge of thestructure coordinates for human beta secretase permits the design and/oridentification of synthetic compounds and/or other molecules which havea shape complementary to the conformation of the human beta secretasebinding site. In particular, computational techniques can be used toidentify or design chemical entities, such as modifiers, agonists andantagonists, that associate with a human beta secretase binding pocketor an beta secretase-like binding pocket. Potential modifiers may bindto or interfere with all or a portion of an active site of human betasecretase, and can be competitive, non-competitive, or uncompetitiveinhibitors; or interfere with dimerization by binding at the interfacebetween the two monomers. Once identified and screened for biologicalactivity, these inhibitors/agonists/antagonists may be usedtherapeutically or prophylactically to block human beta secretaseactivity and, thus, prevent the onset and/or further progression ofAlzheimer's disease. Structure-activity data for analogues of ligandsthat bind to or interfere with human beta secretase or betasecretase-like binding pockets can also be obtained computationally.

The term “chemical entity,” as used herein, refers to chemicalcompounds, complexes of two or more chemical compounds, and fragments ofsuch compounds or complexes. Chemical entities that are determined toassociate with human beta secretase are potential drug candidates. Datastored in a machine-readable storage medium that displays a graphicalthree-dimensional representation of the structure of human betasecretase or a structurally homologous molecule, as identified herein,or portions thereof may thus be advantageously used for drug discovery.The structure coordinates of the chemical entity are used to generate athree-dimensional image that can be computationally fit to thethree-dimensional image of human beta secretase or a structurallyhomologous molecule. The three-dimensional molecular structure encodedby the data in the data storage medium can then be computationallyevaluated for its ability to associate with chemical entities. When themolecular structures encoded by the data is displayed in a graphicalthree-dimensional representation on a computer screen, the proteinstructure can also be visually inspected for potential association withchemical entities.

One embodiment of the method of drug design involves evaluating thepotential association of a known chemical entity with human betasecretase or a structurally homologous molecule, particularly with ahuman beta secretase binding pocket or beta secretase-like bindingpocket. The method of drug design thus includes computationallyevaluating the potential of a selected chemical entity to associate withany of the molecules or molecular complexes set forth above. This methodincludes the steps of: (a) employing computational means to perform afitting operation between the selected chemical entity and a bindingpocket or a pocket nearby the binding pocket of the molecule ormolecular complex; and (b) analyzing the results of said fittingoperation to quantify the association between the chemical entity andthe binding pocket.

In another embodiment, the method of drug design involvescomputer-assisted design of chemical entities that associate with humanbeta secretase, its homologs, or portions thereof. Chemical entities canbe designed in a step-wise fashion, one fragment at a time, or may bedesigned as a whole or “de novo.”

To be a viable drug candidate, the chemical entity identified ordesigned according to the method must be capable of structurallyassociating with at least part of a human beta secretase or betasecretase-like binding pockets, and must be able, sterically andenergetically, to assume a conformation that allows it to associate withthe human beta secretase or beta secretase-like binding pocket.Non-covalent molecular interactions important in this associationinclude hydrogen bonding, van der Waals interactions, hydrophobicinteractions, and electrostatic interactions. Conformationalconsiderations include the overall three-dimensional structure andorientation of the chemical entity in relation to the binding pocket,and the spacing between various functional groups of an entity thatdirectly interact with the beta secretase-like binding pocket orhomologs thereof.

Optionally, the potential binding of a chemical entity to a human betasecretase or beta secretase-like binding pocket is analyzed usingcomputer modeling techniques prior to the actual synthesis and testingof the chemical entity. If these computational experiments suggestinsufficient interaction and association between it and the human betasecretase or beta secretase-like binding pocket, testing of the entityis obviated. However, if computer modeling indicates a stronginteraction, the molecule may then be synthesized and tested for itsability to bind to or interfere with a human beta secretase or betasecretase-like binding pocket. Binding assays to determine if a compound(e.g., an inhibitor) actually interferes with human beta secretase canalso be performed and are well known in the art. Binding assays mayemploy kinetic or thermodynamic methodology using a wide variety oftechniques including, but not limited to, microcalorimetry, circulardichroism, capillary zone electrophoresis, nuclear magnetic resonancespectroscopy, fluorescence spectroscopy, and combinations thereof.

One method for determining whether a modifier binds to a protein isisothermal denaturation. This method includes taking a sample of aprotein (in the presence or absence of substrates) at a fixed elevatedtemperature where denaturation of the protein occurs in a given timeframe, adding the chemical entity to the protein, and monitoring therate of denaturation. If the chemical entity does bind to the protein,it is expected that the rate of denaturation would be slower in thepresence of the chemical entity than in the absence of the chemicalentity. For example, this method has been described in Epps et al.,Anal. Biochem., 292:40-50 (2001).

One skilled in the art may use one of several methods to screen chemicalentities or fragments for their ability to associate with a human betasecretase or beta secretase-like binding pocket. This process may beginby visual inspection of, for example, a human beta secretase or betasecretase-like binding pocket on the computer screen based on the humanbeta secretase structure coordinates listed in Table 1 or othercoordinates which define a similar shape generated from themachine-readable storage medium. Selected fragments or chemical entitiesmay then be positioned in a variety of orientations, or docked, withinthe binding pocket. Docking may be accomplished using software such asQUANTA and SYBYL, followed by energy minimization and molecular dynamicswith standard molecular mechanics forcefields, such as CHARMM and AMBER.

Specialized computer programs may also assist in the process ofselecting fragments or chemical entities. Examples include GRID(Goodford, J. Med. Chem., 28:849-57 (1985); available from OxfordUniversity, Oxford, UK); MCSS (Miranker et al., Proteins: Struct. Funct.Gen., 11:29-34 (1991); available from Molecular Simulations, San Diego,Calif.); AUTODOCK (Goodsell et al., Proteins: Struct. Funct. Genet.,8:195-202 (1990); available from Scripps Research Institute, La Jolla,Calif.); and DOCK (Kuntz et al., J. Mol. Biol., 161:269-88 (1982);available from University of California, San Francisco, Calif.).

Once suitable chemical entities or fragments have been selected, theycan be assembled into a single compound or complex. Assembly may bepreceded by visual inspection of the relationship of the fragments toeach other on the three-dimensional image displayed on a computer screenin relation to the structure coordinates of human beta secretase. Thiswould be followed by manual model building using software such as QUANTAor SYBYL (Tripos Associates, St. Louis, Mo.).

Useful programs to aid one of skill in the art in connecting theindividual chemical entities or fragments include, without limitation,CAVEAT (P. A. Bartlett et al., in “Molecular Recognition in Chemical andBiological Problems,” Special Publ., Royal Chem. Soc., 78:182-96 (1989);Lauri et al., J. Comput. Aided Mol. Des., 8:51-66 (1994); available fromthe University of California, Berkeley, Calif.); 3D database systemssuch as ISIS (available from MDL Information Systems, San Leandro,Calif.; reviewed in Martin, J. Med. Chem., 35:2145-54 (1992)); and HOOK(Eisen et al., Proteins: Struc., Funct., Genet., 19:199-221 (1994);available from Molecular Simulations, San Diego, Calif.).

Human beta secretase binding compounds may be designed “de novo” usingeither an empty binding site or optionally including some portion(s) ofa known modifier(s). There are many de novo ligand design methodsincluding, without limitation, LUDI (Bohm, J. Comp. Aid. Molec. Design.,6:61-78 (1992); available from Molecular Simulations Inc., San Diego,Calif.); LEGEND (Nishibata et al., Tetrahedron, 47:8985 (1991);available from Molecular Simulations Inc., San Diego, Calif.); LeapFrog(available from Tripos Associates, St. Louis, Mo.); and SPROUT (Gilletet al., J. Comput. Aided Mol. Design, 7:127-53 (1993); available fromthe University of Leeds, UK).

Once a compound has been designed or selected by the above methods, theefficiency with which that entity may bind to or interfere with a humanbeta secretase or beta secretase-like binding pocket may be tested andoptimized by computational evaluation. For example, an effective humanbeta secretase or beta secretase-like binding pocket modifier mustpreferably demonstrate a relatively small difference in energy betweenits bound and free states (i.e., a small deformation energy of binding).Thus, the most efficient human beta secretase or beta secretase-likebinding pocket modifiers should preferably be designed with adeformation energy of binding of at most about 10 kcal/mole; morepreferably, at most 7 kcal/mole. Human beta secretase or betasecretase-like binding pocket modifiers may interact with the bindingpocket in more than one conformation that is similar in overall bindingenergy. In those cases, the deformation energy of binding is taken to bethe difference between the energy of the free entity and the averageenergy of the conformations observed when the modifier binds to theprotein.

An entity designed or selected as binding to or interfering with a humanbeta secretase or beta secretase-like binding pocket may be furthercomputationally optimized so that in its bound state it would preferablylack repulsive electrostatic interaction with the target enzyme and withthe surrounding water molecules. Such non-complementary electrostaticinteractions include repulsive charge-charge, dipole-dipole, andcharge-dipole interactions.

Specific computer software is available in the art to evaluate compounddeformation energy and electrostatic interactions. Examples of programsdesigned for such uses include: Gaussian 94, revision C (M. J. Frisch,Gaussian, Inc., Pittsburgh, Pa. (1995)); AMBER, version 4.1 (P. A.Kollman, University of California at San Francisco, (1995));QUANTA/CHARMM (Molecular Simulations, Inc., San Diego, Calif. (1995));Insight II/Discover (Molecular Simulations, Inc., San Diego, Calif.(1995)); DelPhi (Molecular Simulations, Inc., San Diego, Calif. (1995));and AMSOL (Quantum Chemistry Program Exchange, Indiana University).These programs may be implemented, for instance, using a SiliconGraphics workstation such as an Indigo with “IMPACT” graphics. Otherhardware systems and software packages will be known to those skilled inthe art.

Another approach encompassed by this invention is the computationalscreening of small molecule databases for chemical entities or compoundsthat can bind in whole, or in part, to a human beta secretase or betasecretase-like binding pocket. In this screening, the quality of fit ofsuch entities to the binding site may be judged either by shapecomplementarity or by estimated interaction energy (Meng et al., J.Comp. Chem., 13:505-24 (1992)).

This invention also enables the development of chemical entities thatcan isomerize to short-lived reaction intermediates in the chemicalreaction of a substrate or other compound that interferes with or withhuman beta secretase. Time-dependent analysis of structural changes inhuman beta secretase during its interaction with other molecules iscarried out. The reaction intermediates of human beta secretase can alsobe deduced from the reaction product in co-complex with human betasecretase. Such information is useful to design improved analogues ofknown human beta secretase modifiers or to design novel classes ofpotential modifiers based on the reaction intermediates of the humanbeta secretase and modifier co-complex. This provides a novel route fordesigning human beta secretase modifiers with both high specificity andstability.

Yet another approach to rational drug design involves probing the humanbeta secretase crystal of the invention with molecules including avariety of different functional groups to determine optimal sites forinteraction between candidate human beta secretase modifiers and theprotein. For example, high resolution x-ray diffraction data collectedfrom crystals soaked in or co-crystallized with other molecules allowsthe determination of where each type of solvent molecule sticks.Molecules that bind tightly to those sites can then be further modifiedand synthesized and tested for their beta secretase modifier activity(Travis, Science, 262:1374 (1993)).

In a related approach, iterative drug design is used to identifymodifiers of human beta secretase. Iterative drug design is a method foroptimizing associations between a protein and a compound by determiningand evaluating the three-dimensional structures of successive sets ofprotein/compound complexes. In iterative drug design, crystals of aseries of protein/compound complexes are obtained and then thethree-dimensional structures of each complex is solved. Such an approachprovides insight into the association between the proteins and compoundsof each complex. This is accomplished by selecting compounds withinhibitory activity, obtaining crystals of this new protein/compoundcomplex, solving the three-dimensional structure of the complex, andcomparing the associations between the new protein/compound complex andpreviously solved protein/compound complexes. By observing how changesin the compound affected the protein/compound associations, theseassociations may be optimized.

A compound that is identified or designed as a result of any of thesemethods can be obtained (or synthesized) and tested for its biologicalactivity, e.g., inhibition of beta secretase activity.

Pharmaceutical Compositions (Modifiers)

Pharmaceutical compositions of this invention include a potentialmodifier of human beta secretase activity identified according to theinvention, or a pharmaceutically acceptable salt thereof, and apharmaceutically acceptable carrier, adjuvant, or vehicle. The term“pharmaceutically acceptable carrier” refers to a carrier(s) that is“acceptable” in the sense of being compatible with the other ingredientsof a composition and not deleterious to the recipient thereof.Optionally, the pH of the formulation is adjusted with pharmaceuticallyacceptable acids, bases, or buffers to enhance the stability of theformulated compound or its delivery form.

Methods of making and using such pharmaceutical compositions are alsoincluded in the invention. The pharmaceutical compositions of theinvention can be administered orally, parenterally, by inhalation spray,topically, rectally, nasally, buccally, vaginally, or via an implantedreservoir. Oral administration or administration by injection ispreferred. The term parenteral as used herein includes subcutaneous,intracutaneous, intravenous, intramuscular, intra-articular,intrasynovial, intrasternal, intrathecal, intralesional, andintracranial injection or infusion techniques.

Dosage levels of about 0.01 to about 100 mg/kg body weight per day,preferably of about 0.5 to about 75 mg/kg body weight per day of thehuman beta secretase inhibitory compounds described herein are usefulfor the prevention and treatment of human beta secretase mediateddisease. Typically, the pharmaceutical compositions of this inventionwill be administered about 1 to about 5 times per day or alternatively,as a continuous infusion. Such administration can be used as a chronicor acute therapy. The amount of active ingredient that may be combinedwith the carrier materials to produce a single dosage form will varydepending upon the host treated and the particular mode ofadministration. A typical preparation will contain about 5% to about 95%active compound (w/w). Preferably, such preparations contain about 20%to about 80% active compound.

In order that this invention be more fully understood, the followingexamples are set forth. These examples are for the purpose ofillustration only and are not to be construed as limiting the scope ofthe invention in any way.

EXAMPLES Example 1 Crystallization and Structure Determination of HumanBeta Secretase in the C2 Crystal Form

Examples of the crystallization and structure determination of humanbeta secretase in the C2 crystal form are disclosed in U.S. ProvisionalApplication Ser. No. 60/334,648, filed Nov. 30, 2001 and U.S. patentapplication Ser. No. 10/143,723, filed May 10, 2002, now abandoned andentitled “CRYSTALLIZATION AND STRUCTURE DETERMINATION OF BACE AND/ORBACE-LIKE PROTEINS”.

Expression, Purification, and Crystallization

A BACE construct, pQE70-Met-Arg-Gly-Ser-Phe-Val-Glu- . . .Thr-Asp-Glu-Ser-Arg-Ser-(His)₆ (see SEQ ID NO:1) referred to aspQE70-BACE was cloned and expressed as inclusion bodies. Inclusionbodies obtained from 40 liters of cell culture were washed one time in700 ml of 10 mM TRIS buffer, pH 8.12, 1 mM EDTA (TE). The inclusionbodies were extracted with 400 ml 7.5 M urea, 100 mM AMPSO, 1 mMglycine, 1 mM EDTA, and 100 mM β-Mercaptoethanol (BME), pH 10.5-10.8.After centrifugation, the protein concentration of the supernatant wasadjusted by dilution with the above buffer to read ˜5.0 at A₂₈₀. Theprotein was then diluted with 7.5 M urea, 100 mM AMPSO, 1 mM glycine, 1mM EDTA, and the BME concentration adjusted to 10 mM by the addition ofBME to read an A₂₈₀˜0.5 and a pH=10.5-10.8. The solution wascentrifuged. Analysis of the sample in 7.5 M urea by SDS-PAGE revealedBACE as the major component of the solubilized inclusion bodies. BACEmigrated as a band of Mr˜45,000. Refolding was carried out by a 20-25fold dilution with cold water (4-15° C.). Upon dilution, the pH droppedautomatically to 9.5-10.2. The sample was then allowed to rest in thecold room. Activity assays were performed daily to monitor proteinrefolding. Results from various experiments indicated that maximalactivity was usually reached after 4-5 weeks.

Prior to purification, the pH of the refolded protein was lowered fromabout 10 to 8.5 with HCl. The solution was loaded onto three 50 mlQ-Sepharose columns (Pharmacia Biotech XK 50). The columns werepre-equilibrated in 0.4 M Urea, 10 mM AMPSO, pH 8.5. After refoldedprotein was loaded onto the columns, they were washed with 500 ml of 0.4M Urea, 10 mM TRIS, pH 8.2. The columns were eluted with 180-245 ml of0.75 M NaCl in 0.4 M Urea 10 mM TRIS buffer, pH 8.2. The eluates werethen dialyzed versus 20 mM HEPES, pH 8.0. The samples were then removedfrom dialysis and dropped into 1 M NaMES, pH 5.7 (0.1 M finalconcentration). After centrifugation (20Kxg) the supernatant was droppedinto 1 M Na-acetate, 1 M NaMES, pH 5.0 (0.2 M Na-acetate, 0.28 M Na-MESwas the final concentration). No precipitation was observed at thisstep. This solution was then applied to a 15 ml affinity columnequilibrated at the same pH. The column was washed with 6 column volumesof 20 mM sodium acetate buffer pH 4.5, 150 mM NaCl. BACE was eluted atpH 8.5 using about 50 ml of 0.1 M borate buffer. The resin had beencross-linked with the synthetic peptide shown in FIG. 2. This final stepremoved any residual contaminants. From 40 liter of E. coli cellculture, the amount of protein obtained was 137 mg of highly purifiedpQE70-BACE construct. Purified pQE70-BACE was dialyzed into 100 mMNaBorate pH 8.5.

In an effort to reduce non-specific aggregation, the enzyme wasconcentrated in the presence of the inhibitor. The purpose of this wastwo-fold: First, the compound would bind active molecules forming ahomogenous population, and second, the complex, once locked in placewould then not be able to contribute to non-specific aggregation. Theconcentration of the stock protein solution was determined andmultiplied by 2.4. This calculation provides the excess inhibitor to beadded to the dilute protein sample before the concentration. Theappropriate amount of 50 mM inhibitor stock solution (in 100% DMSO) wasadded, and the solution was incubated on ice 30 minutes beforeconcentration. A 30K MWCO (Molecular Weight Cut-Off) Ultrafree-4concentrator (Millipore, Bedford, Mass.) was pretreated with 2.0 ml ofthe following solution: 20 mM Hepes pH 7.8, 20% Glycerol, 5% PEG 8000,0.1 M NaCl. The sample was spun at 3810 rpm (3000×g) in an SH-3000 rotorin 10 minute increments until desired volume is achieved. The membranewas rinsed with 2×1.0 ml 20 mM Hepes pH 7.8. The first aliquot ofprotein:compound mix was added to the concentrator and spun as aboveuntil ½ the volume remained. The concentrator was gently inverted to mixthe protein and another aliquot was added. The above procedure wasrepeated until all of the unconcentrated protein:compound mix was in theconcentrator. At this point, the sample was gently concentrated until afinal volume was reached that yielded approx. 8-10 mg/ml concentration.This concentrated sample was used for co-crystallization studies. It wasalso determined that concentrating the protein in the absence ofinhibitor to a concentration of 10-13 mg/ml and then subsequently addinginhibitor to the protein provided a protein sample that wouldcrystallize albeit at a slower rate.

Sparse matrix screening of pQE70-BACE in the presence of the inhibitorshown in FIG. 1 at 20° C. in the hanging drop vapor diffusion method wasperformed with the commercially available Wizard I screen (EmeraldBiostructures, Bainbridge Island, Wash.) and Hampton I screen (HamptonResearch, Laguna Nigel, Calif.). A shower of microcrystals was observedin Wizard I screen condition 45 (20% PEG 3000 (precipitant), 0.1 Msodium acetate pH 4.5 (buffer)) and Hampton I screen condition 37 (8%PEG 4000 (precipitant), 0.1 M sodium acetate pH 4.6 (buffer)). Firstround optimization experiments, pQE70-BACE produced crystals in 0.1 Msodium acetate pH 4.5-5.6 (buffer) and PEG 2000, 3000, 4000, and 8000(precipitants) at 20° C. Initial crystals grown in PEG 3000 consisted ofrod clusters and single rod shaped crystals with a large depletion. Acrystal grown in 16% PEG 3000 (precipitant) and 0.1 M sodium acetate pH4.6 (buffer) at 20° C., with an approximate size of 0.6×0.15 mm,diffracted to 1.7 Å at the Argonne National Laboratory. The cryogenicsolution for this crystal consisted of synthetic mother liquor based onthe well solution and glycerol: 16% PEG 3000, 0.1 M sodium acetate pH4.6, and 5-30% glycerol (cryoagent) in 5% increments. The cryogenicsolutions were added stepwise in 5-minute increments with increasingpercentages of glycerol. The crystal was looped out and flash frozen inliquid nitrogen. The crystal contains one molecule per asymmetric unitwith cell dimensions of a=73.1 Å, b=105.1 Å, c=50.5 Å, α=90°, β=94.8°,γ=90° in space group C2.

Second round optimization also resulted in large single crystals, withan approximate size of 0.3−0.4×0.12-0.2 mm, in 4-6% PEG 4000 and PEG8000 (precipitants), 0.1 M sodium acetate pH 4.6-5.6 (buffers) at 20° C.The typical cryogenic solution for these crystals consisted of onepercentage higher of the same PEG condition found in the well motherliquor of the crystal, 0.1 M sodium acetate (pH of the mother liquor),and 25% glycerol. The cryogenic solutions were added every 5 minuteswith stepwise additions of 0.1 μl, 0.25 μl, 0.50 μl, 1.0 μl, and 2.0 μl.After a one hour soak, crystals were looped out and flash frozen inliquid nitrogen. These crystals have the same unit cell as the pET11acrystals (a=81 Å, b=103 Å, c=100 Å, α=β90, β=105°) with a space group ofP2₁ and three molecules per asymmetric unit but diffracted to lowerresolution (2.5-2.8 Å) than the C2 crystal form, discussed above.

Third round optimization revealed that the percentage of PEG(precipitant) was the critical component required to distinguish betweenthe C2 and P2₁ crystal forms. The greater the percentage of precipitantpresent, the more dehydrated the crystals become resulting in higherresolution diffraction (the solvent content in the P2₁ crystals is 54%compared to the 42% solvent content for the C2 crystals).Crystallization in 8% PEG or less reproducibly gave the lower resolutionP2₁ crystal form while crystallization in 16% PEG or more (up to 45%PEG) reproducibly gave the higher resolution C2 crystal form.Alternative PEGs such as PEG 200, PEG 350 MME, PEG 400, PEG 550 MME, PEG750 MME, PEG 1000, PEG 2000, PEG 2000 MME, PEG 3000, PEG 4000, PEG 8000also produce suitable crystals when used with pQE70-BACE. The buffer forcrystallizing with the different forms of PEG was 0.1 M sodium acetatepH 4.6-5.6 at a temperature of 20° C. No additional salt was requiredfor crystallization. In addition, streak seeding at the stage of settingup the hanging drops aided crystal growth. A range of proteinconcentration from 2 mg/ml to 13 mg/ml has proven useful in preparingcrystals.

X-Ray Diffraction Characterization

Initial data collection was carried out on home source X-rays using aRigaku RUH3R X-ray generator (with osmic confocal mirros) and a R-axisIV ++ detector (Molecular Structure Corporation, The Woodlands, Texs).Initial analysis of the crystals revealed 1.9 Å diffraction on the homesource. The same crystal was refrozen and transported to the synchrotronfor subsequent data collection at the Advanced Photon Source (Argonne,Ill.) at beamline 17-ID. Using synchrotron radiation, the crystaldiffracted to 1.7 Å resolution. Crystals were of the space group C2 withcell dimensions of a=73.1 Å, b=105.1 Å, c=50.5 Å, α=90°, β=94.8°, γ90°.The Matthews coefficient for these crystals assuming that there is onemolecule in the asymmetric unit is 2.1 Å/Da with 42% solvent. Thestructure determination (see below) revealed the presence of electrondensity in the active site appropriate for the inhibitor shown in FIG.1.

Molecular Replacement

The structure was solved by molecular replacement. A solution wasdetermined using AMORE (Navaza, Acta Cryst., D50: 157-63 (1994);Collaborative Computational Project N4, Acta Cryst. D50:760-3 (1994)) byutilizing a previously solved structure of human BACE produced in E.coli from the pET11a vector. The initial rotation solution gave a singlestrong peaks of 16.9σ. A translation search in space group C2 resultedin a correlation coefficient of 57.2 with an R-factor of 38.3% to 4 Åresolution. The high correlation coefficient and low R-factor suggestedthat the entire protein contents of the unit cell had been correctlyidentified; therefore, the search for additional molecules wasabandoned.

TABLE 6 Data collection statistics for 1.9 Å resolution data set ofHuman BACE derived from E. coli pQE70-BACE (C2 crystal form) producedprotein used for refinement (data collected at λ 1.54 Å on home sourceX-rays). Data was processed with D*trek. Rmerge vs Resolution ResolutionAverage Num Num Num Num <<I>/ ChiSq Rmerge Rmerge range counts obs rejsovlps mults <sig>> norm shell cumul 19.97-4.08  33303 11727 46 116662975 10.3 0.35 0.039 0.039 4.08-3.25 27063 11593 43 11536 2913 9.6 0.480.048 0.043 3.25-2.84 11447 11354 44 11298 2839 8.1 0.75 0.066 0.0472.84-2.58 6285 11441 77 11343 2852 6.8 0.99 0.080 0.049 2.58-2.39 418811239 98 11112 2785 5.8 1.22 0.097 0.052 2.39-2.25 3250 11130 123 109752753 5.1 1.30 0.111 0.054 2.25-2.14 2298 11102 164 10893 2732 4.4 1.400.131 0.056 2.14-2.05 1621 9532 111 9355 2350 3.7 1.41 0.149 0.0572.05-1.97 1278 6190 95 6012 1527 3.6 1.26 0.147 0.058 1.97-1.90 798 413147 4004 1026 3.1 1.16 0.174 0.058 19.97-1.90  10512 99439 848 9819424752 6.5 1.00 0.058 0.058 Resolution Calc Percent of reflectionsmeasured N times, N = % Comp % Comp range unique 0 1 2 3 4 5-8 9-12 >12shell cumul 19.97-4.08  3039  1.6 0.5 5.3 0.6 88.5 3.5 0.0 0.0 98.4 98.44.08-3.25 3011  2.8 0.5 3.2 0.8 89.5 3.3 0.0 0.0 97.2 97.8 3.25-2.842964  3.8 0.4 2.4 0.6 89.3 3.5 0.0 0.0 96.2 97.3 2.84-2.58 3013  4.6 0.72.5 0.5 88.4 3.3 0.0 0.0 95.4 96.8 2.58-2.39 2974  5.4 1.0 2.0 0.7 87.23.8 0.0 0.0 94.6 96.4 2.39-2.25 2963  6.0 1.1 2.1 0.7 86.4 3.7 0.0 0.094.0 96.0 2.25-2.14 2974  6.6 1.5 2.1 0.7 85.4 3.7 0.0 0.0 93.4 95.62.14-2.05 2986 19.1 2.2 1.4 1.6 72.7 2.9 0.0 0.0 80.9 93.8 2.05-1.972985 46.1 2.8 1.7 2.0 45.2 2.2 0.0 0.0 53.9 89.3 1.97-1.90 2978 62.9 2.71.2 2.2 29.9 1.2 0.0 0.0 37.1 84.1 19.97-1.90  29887 15.9 1.3 2.4 1.076.3 3.1 0.0 0.0 84.1 84.1 Percent of reflections measured AT LEAST Ntimes, N = Resolution range 13 9 5 4 3 2 1 19.97-1.90 0.0 0.0 3.1 79.480.4 82.8 84.1

TABLE 7 Data collection statistics for 1.7 Å resolution dataset of HumanBACE derived from E. coli pQE70-BACE (C2 crystal form) produced proteinused for the intial molecular replacement solution (data collected at λ1.0000 Å at APS, 17-ID). Data was processed with HKL2000. Summary ofreflections intensities and R-factors by shells R linear = SUM (ABS(I −<I>))/SUM (I) R square = SUM ((I − <I>) ** 2)/SUM (I ** 2) Chi**2 = SUM((I − <I>) ** 2)/(Error ** 2 * N/(N − 1))) In all sums singlemeasurements are excluded Shell Lower Upper Average Average Norm. LinearSquare limit Angstrom I error stat. Chi**2 R-fac R-fac 50.00  3.6637330.4 701.7 364.7 2.455 0.039 0.045 3.66 2.91 18841.1 418.8 278.72.645 0.054 0.059 2.91 2.54 7573.0 231.6 188.0 2.262 0.069 0.074 2.542.31 4744.8 196.2 174.2 1.911 0.085 0.088 2.31 2.14 3766.7 199.8 185.11.655 0.097 0.099 2.14 2.02 2682.2 195.6 186.9 1.324 0.115 0.110 2.021.91 1738.0 182.7 178.3 1.079 0.143 0.131 1.91 1.83 1038.1 170.3 168.20.833 0.183 0.159 1.83 1.76 665.1 166.9 165.9 0.691 0.235 0.200 1.761.70 625.6 219.6 219.0 0.608 0.220 0.205 All reflections 8296.7 272.2211.6 1.651 0.060 0.052 Shell I/Sigma in resolution shells: Lower Upper% of reflections with I/Sigma less than limit limit 0 1 2 3 5 10 20 >20total 50.00  3.66 0.1 0.3 0.5 0.6 0.7 1.7  4.2 93.9 98.0 3.66 2.91 0.40.9 1.4 2.1 3.6 7.3 16.0 82.5 98.6 2.91 2.54 1.0 2.4 3.8 5.4 8.0 16.134.0 64.0 98.0 2.54 2.31 1.6 3.7 6.8 9.4 15.0 27.8 51.2 46.5 97.7 2.312.14 2.2 5.6 9.0 12.7 20.3 36.3 62.3 34.8 97.1 2.14 2.02 2.3 8.1 14.319.6 29.6 49.6 75.4 21.3 96.7 2.02 1.91 4.0 12.6 20.9 28.6 42.2 63.584.6 11.7 96.3 1.91 1.83 6.8 20.8 32.8 42.2 56.1 75.9 89.9 5.4 95.3 1.831.76 7.6 25.7 40.6 51.6 64.0 80.2 88.7 2.0 90.6 1.76 1.70 5.5 19.7 33.243.6 54.5 64.1 67.7 0.3 68.0 All hkl 3.1 10.0 16.3 21.5 29.3 42.2 57.336.4 93.7Model Building and Refinement

Further rigid body refinement of the model in CNX (MolecularSimulations, Inc) followed by minimization refinement gave an R-factorof 37.5% and a Free R-factor of 40.1% to 4.0 Å with an overall B-factorof 25.0 Å². Minimization and B-factor refinement led to a R-factor of30.7% and a Free R-factor of 33.4%. The subsequent availability of ahigher resolution data set to 1.7 Å afforded the opportunity to continuethe high resolution refinement of the structure. The refinement againsthigher resolution data was initiated with a rigid body refinementfollowed by minimization and B-factor refinement leading to a R-factorof 29.6% and a Free R-factor of 30.7%. Including a round of simulatedannealing refinement (A. T. Brunger, A. Krukowski, J. W. Erickson ActaCryst A 46:585-93, (1990)) and minimization led to an improved R-factorof 26.3% and a Free R-factor of 28.3%. During each cycle of refinement abulk solvent correction was incorporated (J. S. Jiang & A. T. Brunger,J. Mol. Biol. 243:100-15 (1994)). Progress of the refinement wasmonitored by a decrease in both the R-factor and Free R-factor.

At this point, inspection of the electron density map within the activesite revealed electron density that was unaccounted for by the proteinmodel and consistent with the shape of the inhibitor shown in FIG. 1that was present in the crystallization conditions. Model building wasdone using the program CHAIN (J. S. Sack, Journal of Molecular Graphics6:224-5 (1988)) and LORE (B. C. Finzel, Meth. Enzymol. 277:230-42(1997)). Rebuilding of the model and the addition of water moleculesinto the model using the 1.7 Å resolution map afforded the opportunityfor further cycles of refinement (including the inhibitor) givingimprovement of the R-factor to 20.6% and a Free R-factor of 23.1%. Themodel includes three residues from the N terminal pro-region (61P-63P),residues 1-157, 165-309, 317-386. Loops for residues 158-164 and 310-316were disordered in the electron density and therefore have been omittedfrom the model.

TABLE 8 Refinement Statistics for structure of Human BACE from pQE70 (C2crystal form). R-factor Free R-factor No. of reflections 20-1.70 Å F ≧2σ 0.206 0.231 37857 Bonds (Å) Angles (°) r.m.s deviation from idealgeometry 0.006 1.4 Number of atoms Average B-factor Protein 2982 24.6Waters 370 35.5 Ligand 41 18.7 Total 3393 25.8

Example 2 Crystallization and Structure Determination of Human BetaSecretase in the C222₁ Crystal Form

Expression, Purification, and Crystallization

A BACE construct, pQE70-Met-Arg-Gly-Ser-Phe-Val-Glu- . . .Thr-Asp-Glu-Ser-Arg-Ser-(His)₆ (see SEQ ID NO:1) referred to aspQE70-BACE was cloned and expressed as inclusion bodies. Inclusionbodies obtained from 40 liters of cell culture were washed one time in700 ml of 10 mM TRIS buffer, pH 8.12, 1 mM EDTA (TE). The inclusionbodies were extracted with 400 ml 7.5 M urea, 100 mM AMPSO, 1 mMglycine, 1 mM EDTA, and 100 mM β-Mercaptoethanol (BME), pH 10.5-10.8.After centrifugation, the protein concentration of the supernatant wasadjusted by dilution with the above buffer to read ˜5.0 at A₂₈₀. Theprotein was then diluted with 7.5 M urea, 100 mM AMPSO, 1 mM glycine, 1mM EDTA, and the BME concentration adjusted to 10 mM by the addition ofBME to read an A₂₈₀˜0.5 and a pH=10.5-10.8. The solution wascentrifuged. Analysis of the sample in 7.5 M urea by SDS-PAGE revealedBACE as the major component of the solubilized inclusion bodies. BACEmigrated as a band of Mr˜45,000. Refolding was carried out by a 20-25fold dilution with cold water (4-15° C.). Upon dilution, the pH droppedautomatically to 9.5-10.2. The sample was then allowed to rest in thecold room. Activity assays were performed daily to monitor proteinrefolding. Results from various experiments indicated that maximalactivity was usually reached after 4-5 weeks.

Prior to purification, the pH of the refolded protein was lowered fromabout 10 to 8.5 with HCl. The solution was loaded onto three 50 mlQ-Sepharose columns (Pharmacia Biotech XK 50). The columns werepre-equilibrated in 0.4 M Urea, 10 mM AMPSO, pH 8.5. After refoldedprotein was loaded onto the columns, they were washed with 500 ml of 0.4M Urea, 10 mM TRIS, pH 8.2. The columns were eluted with 180-245 ml of0.75 M NaCl in 0.4 M Urea 10 mM TRIS buffer, pH 8.2. The eluates werethen dialyzed versus 20 mM HEPES, pH 8.0. The samples were then removedfrom dialysis and dropped into 1 M NaMES, pH 5.7 (0.1 M finalconcentration). After centrifugation (20Kxg) the supernatant was droppedinto 1 M Na-acetate, 1 M NaMES, pH 5.0 (0.2 M Na-acetate, 0.28 M Na-MESwas the final concentration). No precipitation was observed at thisstep. This solution was then applied to a 15 ml affinity columnequilibrated at the same pH. The column was washed with 6 column volumesof 20 mM sodium acetate buffer pH 4.5, 150 mM NaCl. BACE was eluted atpH 8.5 using about 50 ml of 0.1 M borate buffer. The resin had beencross-linked with synthetic peptide shown in FIG. 2. This final stepremoved any residual contaminants. From 40 liter of E. coli cellculture, the amount of protein obtained was 137 mg of highly purifiedpQE70-BACE construct. Purified pQE70-BACE was dialyzed into 100 mMNaBorate pH 8.5.

Crystallization Conditions for Unliganded BACE. Exploration of theoptimal PEG conditions for growth of E. coli pQE70 BACE-1 co-crystalsled to identification of a crystal structure of pQE70 without ligands(e.g., modifiers or inhibitors) bound to the active site. In an effortto repeat the concentration and conditions originally used to obtain theunliganded BACE-1 crystals, the molarity of the dilute protein samplewas calculated in order to determine the amount of excess ligand (2.4×in 100% DMSO) that usually would have been added. The protein wasincubated on ice with 2.4 excess of 100% DMSO for one hour. AnUltrafree-4 30,000 molecular weight cutoff centrifugal filter and tube(Millipore, Bedford, Mass.) was rinsed with 2.5 ml prep bufferconsisting of 20 mM HEPES, pH 7.8, 20% glycerol, 5% PEG 8000, and 0.1 Msodium chloride. After incubation, the protein was added to thecentricon and spun at 3500 rpm with a SH-3000 rotor at 5 minuteintervals until the desired volume that provided a 6-8 mg/ml sample wasreached.

Further experimentation demonstrated that concentration of pQE70 in thepresence of DMSO as well as addition of DMSO to concentrated proteinresults in crystal growth in less than a week. It was also determinedthat streak seeding was preferred for crystal growth. A cat whisker wasused to seed at setup with a thousand fold dilution of seed stock. Thecrystallization conditions were in the range of 24-39% PEG 200 andsodium acetate pH 5.0-5.6. The crystal morphology included single rodswith an approximate size of 0.25×0.15 mm. The crystals were loopeddirectly from the drop and flash frozen in liquid nitrogen. A crystalwas diffracted to 1.75 Å at the Argonne National Laboratory. The crystalcontained one molecule per asymetric unit with cell dimensions of a=75,b=104, c=100, and α=γ=β=90° in space group is C222₁.

Soaking Protocol to Produce Liganded BACE Crystals

Soaking experiments were performed using pQE70 crystals that were atleast 0.15×0.1×0.05 mm. The well conditions for crystallization traysset up for soaking experiments consisted of 30% PEG 200, 100 mM sodiumacetate pH 5.4 or 5.6. Ligand stabilization solution consisted of 33%PEG 200, 50 mM sodium acetate, pH 5.6, 50 mM sodium borate, pH 8.5, and1-10 mM compound. The soaking solution was added over a two-hour period.After overnight incubation at 20° C., the crystals were looped out ofthe drop since both the growth and stabilization solution conditionswere cryogenic.

Specifically, soaking experiments using the inhibitor shown in FIG. 1(IC₅₀=1 nM) and the inhibitor shown in FIG. 7 (IC₅₀=8.8 μM) were used todefine the preferred parameters for soaking inhibitors into unligandedpQE70 crystals. Using the data from experiments with the figure shown inFIG. 1, it was determined that a 2 mM overnight soak would be sufficientfor high potency inhibitors (IC₅₀<100 nM). Using data from experimentswith the inhibitor shown in FIG. 7, it was determined that a 5 mMovernight soak would provide a protein-inhibitor complex with moderateto low potency inhibitors (IC₅₀>100 nM).

However, some ligands may not successfully be soaked into unligandedcrystals using these parameters. For such cases, the soak would berepeated using a higher concentration of ligand. If soaking under theseconditions does not provide a protein-ligand complex, co-crystallizationof a ligand with the pQE70 construct may be used to provide theprotein-ligand complex.

X-Ray Diffraction Characterization

The initial crystals diffracted to 1.75 Å using synchrotron radiation atthe Advanced Photon Source (Argonne, Ill.) at beamline 17-ID (operatedby the Industrial Macromolecular Crystallography Consortium). Crystalswere of the space group C222₁ with cell dimensions of a=75.0 Å, b=104.0Å, c=100.4 Å, α=β=γ=90. The Matthews coefficient for these crystals,assuming that there is one molecule in the asymmetric unit, is 2.1 Å/Dawith 42% solvent.

Molecular Replacement

The structure was solved by molecular replacement. A solution wasdetermined using AMORE (Navaza, Acta Cryst., D50:157-63 (1994);Collaborative Computational Project N4, Acta Cryst. D50:760-3 (1994)) byutilizing the coordinates from the structure of BACE in the C2 crystalform. The initial rotation solution gave a single strong peak of 11.5σ.A translation search in space group C222₁ resulted in a correlationcoefficient of 59.2 with an R-factor of 38.1%. Further refinement ofthis solution in AMORE resulted in a correlation coefficient of 64.8with an R-factor of 35.2% to 4 Å resolution. The high correlationcoefficient and low R-factor suggested that the entire protein contentsof the unit cell had been correctly identified; therefore, the searchfor additional molecules was abandoned.

TABLE 9 Data collection statistics for stucture of unliganded E. coliproduced BACE (pQE70 construct) Summary of reflections intensities andR-factors by shells R linear = SUM (ABS(I − <I>))/SUM (I) R square = SUM((I − <I>) ** 2)/SUM (I ** 2) Chi**2 = SUM ((I − <I>) ** 2)/(Error **2 * N/(N − 1))) In all sums single measurements are excluded Shell LowerUpper Average Average Norm. Linear Square limit Angstrom I error stat.Chi**2 R-fac R-fac 50.00 3.77 9478.7 153.1 90.7 1.401 0.035 0.043 3.772.99 5425.8 99.8 70.2 1.133 0.039 0.045 2.99 2.61 2491.1 53.6 42.5 0.9350.044 0.049 2.61 2.38 1530.8 41.7 36.0 0.782 0.051 0.054 2.38 2.201238.1 44.3 40.1 0.757 0.061 0.062 2.20 2.07 967.7 41.6 38.8 0.845 0.0870.091 2.07 1.97 743.9 42.1 40.1 0.856 0.120 0.125 1.97 1.89 1552.1 93.989.0 1.683 0.170 0.177 1.89 1.81 283.4 45.4 44.9 0.632 0.244 0.222 1.811.75 266.5 78.0 77.6 0.728 0.244 0.263 All reflections 2487.4 68.0 54.90.950 0.051 0.048 Shell I/Sigma in resolution shells: Lower Upper % ofreflections with I/Sigma less than limit limit 0 1 2 3 5 10 20 >20 total50.00 3.77 0.9 1.6 1.7 1.9 2.5 3.9 6.5 84.3 90.8 3.77 2.99 0.5 1.0 1.31.8 2.8 4.9 9.9 71.2 81.1 2.99 2.61 0.4 1.3 2.2 3.2 5.1 10.5 21.4 78.6100.0 2.61 2.38 1.0 2.5 4.0 5.3 8.6 16.2 31.4 68.6 100.0 2.38 2.20 2.33.2 4.2 5.2 7.3 11.8 20.3 26.9 47.2 2.20 2.07 2.7 5.4 8.9 11.9 17.9 30.655.4 44.6 100.0 2.07 1.97 1.8 6.1 10.3 14.8 22.6 41.3 69.3 30.7 100.01.97 1.89 7.3 9.0 10.5 12.0 14.6 20.4 31.7 12.2 43.9 1.89 1.81 11.7 21.931.2 39.5 54.2 76.7 94.2 4.7 98.9 1.81 1.75 3.9 16.4 31.3 43.3 59.1 72.578.3 0.7 79.0 All hkl 3.2 6.8 10.5 13.8 19.3 28.7 41.5 42.7 84.2 ShellSummary of observation refundancies: Lower Upper % of reflections withgiven No. of observations limit limit 0 1 2 3 4 5-6 7-8 9-12 13-19 >19total 50.00 3.77 9.2 2.0 2.6 3.4 11.2 10.0 61.6 0.0 0.0 0.0 90.8 3.772.99 18.9 1.0 1.0 2.3 6.5 10.7 59.5 0.0 0.0 0.0 81.1 2.99 2.61 0.0 0.10.4 1.9 6.4 13.6 77.6 0.0 0.0 0.0 100.0 2.61 2.38 0.0 0.0 0.3 1.6 6.015.0 77.0 0.0 0.0 0.0 100.0 2.38 2.20 52.8 1.2 0.8 1.3 3.0 7.9 33.1 0.00.0 0.0 47.2 2.20 2.07 0.0 0.2 0.3 1.9 5.5 18.4 73.7 0.0 0.0 0.0 100.02.07 1.97 0.0 0.0 0.3 1.1 5.5 19.5 73.4 0.2 0.0 0.0 100.0 1.97 1.89 56.12.0 2.4 4.3 5.5 10.1 19.5 0.1 0.0 0.0 43.9 1.89 1.81 1.1 1.5 2.7 5.3 9.634.2 45.1 0.5 0.0 0.0 98.9 1.81 1.75 21.0 17.8 21.3 14.7 11.6 10.9 2.60.0 0.0 0.0 79.0 All hkl 15.8 2.5 3.2 3.8 7.1 15.0 52.5 0.1 0.0 0.0 84.2Model Building and Refinement

Further rigid body refinement of the model in CNX (Accerlys, Inc,www.accerlys.com) followed by minimization refinement gave an R-factorof 42.9% and a Free R-factor of 43.1% to 4.0 Å with an overall B-factorof 25.0 Å². Minimization and B-factor refinement led to a R-factor of32.7% and a Free R-factor of 35.1%. During each cycle of refinement abulk solvent correction was incorporated (Jiang et al., J. Mol. Biol.,243:100-15 (1994)). Progress of the refinement was monitored by adecrease in both the R-factor and Free R-factor. Inspection of theactive site at this stage revealed the absence of any inhibitor electrondensity and the movement of flap residues 66-76 suggested that aligand-free form of the enzyme had been obtained. Model building wasdone using the program CHAIN (Sack, Journal of Molecular Graphics,6:224-25 (1988)) and LORE (Finzel, Meth. Enzymol., 277:230-42 (1997)).The electron density for the main chain of the active site flap wasfairly well resolved, although the electron density for the side chainsof these residues was usually disordered. Further cycles of refinementled to improvement of the R-factor to 23.3% and a Free R-factor of26.8%. Residues 158-163 were disordered in the electron density andtherefore have been omitted from the model. Further details of therefinement are shown in Table 10.

TABLE 10 Refinement Statistics for structure of unliganded E. coliproduced BACE (pQE70 construct) R-factor Free R-factor No. ofreflections 20-1.75 Å F ≧ 2σ 0.234 0.268 33362 Bonds (Å) Angles (°)r.m.s deviation from 0.006 1.6 ideal geometry Number of atoms AverageB-factor Protein 3132 23.6 Waters  205 28.7 Total 3337 23.9

The complete disclosure of all patents, patent applications includingprovisional applications, and publications, and electronically availablematerial (e.g., GenBank amino acid and nucleotide sequence submissions;and protein data bank (pdb) submissions) cited herein are incorporatedby reference. The foregoing detailed description and examples have beengiven for clarity of understanding only. No unnecessary limitations areto be understood therefrom. The invention is not limited to the exactdetails shown and described; many variations will be apparent to oneskilled in the art and are intended to be included within the inventiondefined by the claims.

SEQUENCE LISTING FREE TEXT

-   SEQ ID NO:1 residues for the E. coli expressed recombinant human    beta secretase-   SEQ ID NO:2 synthetic peptide

1. An isolated crystal of unliganded beta secretase polypeptide, havingorthorhombic space group symmetry C222₁ and having a unit cell definedby the dimensions a, b, and c, wherein a is 75 Å, b is 104 Å, c is 100Å, and α=β=γ=90°, wherein said polypeptide consists of SEQ ID NO:1. 2.An isolated crystal of unliganded beta secretase polypeptide, havingorthorhombic space group symmetry C222₁ and having a unit cell definedby the dimensions a, b, and c, wherein a is 75 Å, b is 104 Å, c is 100Å, and α=β=γ=90°, wherein said polypeptide consists of SEQ ID NO:1, withthe proviso that a methionine is replaced with selenomethionine in theamino acid sequence of said polypeptide.